A Beam Forming Network (BFN) constitutes the heart of any array antenna system, i.e. of any antenna system relaying on a set of radiating elements to generate one or more beams. It plays an essential role in different antenna architectures ranging from direct radiating arrays (DRAs) to the vast set of Array-Fed Reflector (AFR) antennas, including Semi-Active Multi-Matrix, Imaging or others configurations.
More specifically, a Beam Forming Network performs the functions of:                in an emitting antenna array, focusing the energy radiated by an array along one or more predetermined directions in space by opportunely phasing and weighting the signals feeding the radiating elements of the array; and        in a receiving antenna array, synthesizing one or more receiving lobes having predetermined directions in space by opportunely phasing and weighting the signals received by the antenna elements of the array.        
Multibeam array antennas find application in communications, remote sensing (e.g. real and synthetic RF instruments such as radars, radiometers, altimeters, bi-static reflectometry and radio occultation receivers for signals-of-opportunity missions, etc.), electronic surveillance and defense systems (e.g. air traffic management and generally moving target indicator radars, electronic support measure and jamming systems for electronic warfare, RF instruments for interference analysis and geo-location, etc.), science (e.g. multibeam radio telescopes), satellite navigation systems (where multibeam antennas can be employed in the user and control segment and could, as well, extend space segment capabilities).
In satellite communication systems, array antennas are required to perform two major classes of coverage:                Multiple Contoured Beams, for the development of broadcasting/multicasting services based on linguistic zones consisting of differently sized and shaped geographical regions; and        Multiple Spots in a cellular-like configuration, especially for point-to-point services making available higher gains and thus relaxing user terminals requirements.        
Telecommunication satellites have an ever-increasing operational lifespan, and business conditions are subject to unpredictable changes. Therefore, there is a need for reconfigurability of multibeam array antennas in order to move beams in space and/or deal with changes in the satellite orbits.
A single-beam BFN for application to an Array Fed Reflector is described in the article of T. E. Sharon, “Beam Forming Networks for mm-Wave Satellite Communications”, Microwave Journal, August 1983. In the described application a spot beam must be generated with scanning flexibility over the Earth coverage. The number of radiating elements to be fed is instantaneously limited to only a portion of the number of elements which constitute the full array, and only amplitude tapering is required. The author shows that the number of amplitude control elements (variable power dividers) can be reduced at the expense of an increased number of switches. The proposed BFN employs a reduced number of variable power dividers to illuminate a cluster of radiating elements, and the position of the cluster can be selected by setting switch positions on the output section.
This configuration was implemented for the beam hopping antenna of the NASA Advanced Communications Technology Satellite (ACTS), as reported in the paper of F. A. Regier “The ACTS multibeam antenna”, IEEE Transaction on Microwave Theory and Techniques, Vol. 40, No 6, pp 1159-1164, June 1992.
Another example of the use of switches to avoid the need of having variable phase shifters in number equal to the number of radiating elements is described by J. L. Butler, “Digital, matrix, and intermediate frequency scanning”, in R. C. Hansen “Microwave Scanning Antennas”, Vol. 3, Academic Press 1966 The author describes a single-beam linear phased array composed by N radiating elements, able to steer a single beam toward N equispaced beam directions.
The above-described BFNs are limited to the generation of a single instantaneous beam, and their only form of reconfigurability is represented by some capability of re-pointing said beam, by continuous steering or discrete hopping.
U.S. Pat. No. 3,255,450 to J. L. Butler describes a fixed multiple-beam BFN based on the use of so-called “Butler matrices”. A Butler matrix is a lossless multiport network having N inputs and N outputs. The excitation of a single input induces equal amplitude signals on all the outputs, with a linear phase progression across the array. Therefore, each of the N input ports give rise to an independent directive beam. The strategy that allows reducing the complexity of the BFN consists in factorizing the whole network in lower order networks. A systematic design procedure for a square network with a number of input/output ports equal to a power of 2 leads to a number of hybrids and of fixed phase shifters equal, respectively, to
            N      2        ⁢          log      2        ⁢                  ⁢    N    ⁢                  ⁢    and    ⁢                  ⁢          N      2        ⁢          (                                    log            2                    ⁢                                          ⁢          N                -        1            )        ,while a non-factorized N×N BFN is composed by ˜N2 power dividers and phase shifters. This complexity reduction is directly equivalent to that obtained, in the field of digital signal processing, by using the Fast Fourier Transform (FFT) algorithm to evaluate the Discrete Fourier Transform (DFT), and indeed the Butler matrix can be seen as an analog implementation of the FFT.
The main limitation of this BFN is its lack of reconfigurability.
A “fully reconfigurable” BFN driving NE antenna elements for generating NB independent beams with maximum flexibility would require NB signal dividers (or combiners, in receiving application) of order 1:NE, NB signal combiners (or dividers, in receiving application) of order NB:1 and, most of all, NE×NB variable attenuators and phase shifters. The complexity of such a network would make it impractical for many applications: simpler solutions retaining sufficient (although not complete) flexibility are therefore necessary.
To take advantage of the complexity savings offered by the Butler's approach, hybrid architectures based on a combination of one or more fixed FFT-like BFNs and of a Reconfigurable BFN with reduced complexity have been proposed. See, for example, U.S. Pat. No. 6,295,026 to C.-H. H. Chan et al. for a reconfigurable BFN adapted to a direct radiating array, and U.S. Pat. No. 5,115,248 to A. Roederer for a reconfigurable BFN for a focused array including multiport amplifiers (known as the “multi-matrix” architecture).
The complexity saving provided by these solutions is, however, insufficient for many applications, particularly in the field of telecommunications.
Moreover, each BFN architecture known from prior art is tailored to a specific antenna architecture (e.g. direct-radiating vs. focused).
An additional drawback of the prior art architectures is that the digital implementations of transmit and receive BFN may be drastically different from each other: this is due to the fact that digital signal dividers and combiners, unlike their analog counterparts, are intrinsically non-reciprocal devices and have completely different structures and implementations.